Golf balls are available as one-piece, two-piece, and multi-layer constructions. One-piece balls lack a separate cover layer and, typically, are formed by molding a polybutadiene based compound in a single step. One-piece balls typically spin at a high rate, have a relatively low linear velocity, and, generally, do not provide the desired driving distance. Consequently, one-piece balls are used most frequently as practice balls or driving range balls.
Two-piece golf balls usually have a core formed of a solid sphere, typically comprising a polybutadiene based compound, and a cover comprising SURLYN.RTM. (DuPont) or other similar ionomeric resin material that encases the core. Ionomeric resin covers, when molded over a solid, one-piece core, are more durable than the softer balata covers on golf balls that are preferred by professional golfers. Two-piece golf balls provide superior distance when driven as compared to one-piece golf balls, and are frequently used by the typical amateur golfer.
Multi-layer golf balls encompass those with cores or covers composed of two or more layers. A golf ball with a dual-layer core comprises a core and a cover which encases the core. The core, in turn, is composed of an inner sphere, referred to as the center, and at least one additional layer, referred to as an outer core layer or a mantle layer, disposed concentrically between the center and the cover. Similarly, golf balls may be constructed with a multi-layer cover. Inner cover layers are formed over the core and are disposed concentrically between the center and the outer cover layer or layers. Generally, the distinction between an inner cover layer and an outer mantle layer has been that the former is often constructed with a thermoplastic material while the latter is usually formed of a thermoset elastomeric material.
Typically, a spherical, solid center is formed by compression or injection molding using a thermal curing process. Outer layers making up the mantle or the layers of the mantle are formed around this center, for example by injection molding of the mantle around the center or by compression molding of two half shells comprising the mantle or mantle layer, around the center. Half shells comprising the mantle or a mantle layer may be formed by injection molding or compression molding. Ultimately, the finished multi-layer core is enclosed within the outermost layer of the ball, i.e., the cover.
Elastomers commonly used in the fabrication of golf ball cores include thermoset rubber compounds, such as polybutadiene, styrene butadiene, isoprene, trans-isoprene and natural rubber. The polymeric components comprising the core, the center, the mantle or the layers of the mantle are chemically bonded using cross linking agents and initiators. The latter materials commonly are thermosensitive peroxide-containing reagents which generate free radicals at the elevated temperatures of the curing process. The cross linking agent is usually a metallic salt of an unsaturated carboxylic acid, often, for example, zinc diacrylate.
Each layer of a dual-layer or multi-layer golf ball core may comprise mixtures that include one or more flexible or rigid polymeric materials, monomers, cross-linkers, thermally induced free radical generating systems, co-cross linkers, stabilizers, antioxidants, accelerators, fillers, dyes and other additives intended to improve the performance of, or manufacturing process for, golf balls.
The composition and properties of each layer of the golf ball can be varied independently in order to alter the physical properties of the finished golf ball. Therefore, the components that make up a golf ball may be varied alone or in combination to create a product that is matched to the skill level of the individual golfer.
Similarly, the durability of a golf ball reflects both the manner in which it is manufactured and the reagents with which it is made. It has been noted that golf ball failures frequently are the result of the formation of cracks formed within the core. In particular, in golf balls with dual-layer or multi-layer cores, it is the mantle layer which often fails. It is believed, although Applicant is not bound by this theory, that cracks within golf ball cores develop from micro-fissures created when the golf ball is driven. Golf balls in which the mantle layer is relatively inflexible as compared to the center, are particularly likely to develop micro-fissures and cracks upon impact. An example of this type of construction is provided by the golf ball disclosed in U.S. Pat. No. 5,601,502. The ball of the '502 patent is a three-part ball formulated with a decreased level of the cross-linking agent in the center and an increased level of the cross-linking agent in the mantle. The amount of rubber in the center, therefore, is increased relative to that in the mantle, yielding a relatively soft-centered golf ball. Although this ball has a high impact resilience, it may be less durable.
Core compositions generally include a blend of materials designed to achieve a balance between elasticity and rigidity. The former property enhances ball control and "feel" while the latter provides driving distance and strength. Components used to provide rigidity to core compositions have included ionomeric polymers as well as non-polar polymers, examples of which are disclosed in the following U.S. Patents.
U.S. Pat. No. 5,253,871 discloses the use of amide block polyethers in the construction of the mantle layer. The weak elastic modulus of the amide block polyethers results in a tendency for the balls to deform, and is corrected by adding ionomers to the blend. The mantle layer of the golf ball disclosed by the reference comprises at least 10% amide block polyether and is combined with one or more ionomers.
U.S. Pat. No. 5,439,227 discloses the use of polyether ester thermoplastic elastomers for the construction of mantle layers which can be injection molded. However, these materials require the addition of an ethylene-methacrylate copolymer at a level of up to 50 phr to provide the desired balance of hardness and impact resilience.
U.S. Pat. No. 5,681,898 discloses a mantle layer in which the polybutadiene elastomer is blended with a polymer formed between n-butyl acrylate and ethylene-methacrylate to achieve the desire combination of properties. The polymer is present in this layer at a level of at least 50 phr.
U.S. Pat. Nos. 3,384,612 and 3,421,766 disclose one-piece golf balls constructed with blends of an elastomer and an ionomer. The latter patent further discloses the addition of a second rosinous material, preferably a styrene copolymer or other non-polar material, in order to achieve the desired balance of properties in the resulting golf ball. The latter polymers were added at levels between 5 and 50 phr.
The material blends in these examples generally are combinations of elastomers and more rigid polymers. The latter materials frequently are ionomeric resins, which are ionic copolymers of an olefin and an unsaturated carboxylic acid in which at least a portion of the carboxylic acid groups have been neutralized with a metal ion. Polymers comprising acrylate monomers that have not been neutralized will, nevertheless, behave as ionomers as a result of a phenomenon referred to as "ion hopping." Blending these materials with, for example, the common cross-linking agent zinc diacrylate, would be expected to result in an exchange between the acidic protons of the carboxylic moieties of the polymer and the metal ions of the cross-linking agent.
One consequence of the ionic properties of these materials is that they are immiscible with many other, softer polymers in the same manner as oil and water. Materials that are sufficiently different chemically will not form strong interactions when they are mixed. Blends of polymers that are not compatible will result in the formation of separate phases, which diminish the overall mechanical strength of the composition. Golf ball layers formed from non-compatible mixtures will therefore form micro-fissures on impact that eventually develop into cracks. Consequently golf ball layers fabricated with these mixtures, and golf balls comprising these layers, will not be durable.
Non-polar polymers are also combined with elastomers to provide a low-density filler as well as to provide strength to the blend. Examples of the former use include U.S. Pat. Nos. 5,368,304, 5,580,057 and 5,733,207 which disclose two-piece, low-spin golf balls which are larger than usual but which still conform to the U.S.G.A. standards for maximum weight. Each of these disclose the addition of polypropylene powder to core formulations at levels of up to 25 phr, where the polypropylene serves as a low density filler that also increases ball compression. U.S. Pat. Nos. 3,756,607 and 5,482,285 also disclose non-polar polymers, including polyethylene, as filler materials chosen for their low density. The former patent, directed toward one-piece molded balls, includes polyethylene at a level between 2 and 40 phr. The latter patent, directed toward three-piece balls, discloses the use of resin foams as low-density filler materials in the "outer core" or mantle layer.
"Gum plastics," including polyvinyl chloride, ABS polymers (acrylonitrile-butadiene-styrene) and ethylene-propylene based copolymers are also blended with an elastomer to yield one-piece golf balls having properties similar to that of prior art balls but which can be manufactured more easily and less expensively as disclosed in U.S. Pat. No. 3,666,272. These materials are present at 15 to 60 phr of butadiene.
Polyethylene, often in the form of discrete particles dispersed within the elastomer, is also used as an impact modifier in one-piece and two-piece golf balls, providing increased strength and durability as disclosed in U.S. Pat. Nos. 3,572,721 and 4,141,559. U.S. Pat. No. 3,478,132 discloses the use of finely divided particles of high molecular weight polyethylene which do not melt to form a continuous phase when mixed with the elastomeric materials, polybutadiene and polyisoprene, used for the formation of a one-piece golf ball. The polyethylene, present at a level of 35 phr of the elastomeric materials, also did not melt during the molding process. The polyethylene therefore was essentially, simply dispersed within the elastomer as a second phase, providing an impact modifier that improved the impact resistance of the golf ball.
As exemplified by U.S. Pat. No. 3,478,132 and as noted above, non-polar polyolefin additives would not be expected to form compatible mixtures with the elastomers of this invention. The lack of compatibility would be expected to result in phase separations conducive to the formation of micro-fissures and the development of cracks in golf ball cores and the mantle layer or layers of multi-layer core golf balls formulated with these materials.
Consequently, there is a need for compatible materials and compositions that can be formulated into golf ball cores, and the mantle layer or layers of multi-piece golf ball cores, which will enhance the durability of the core and the golf ball comprising that core. The present invention discloses such a composition and a method for providing more durable golf ball cores and golf balls.